1. Field of the Invention
The present invention relates to a one-component magnetic toner with high resistivity for use in electrophotography for developing latent electrostatic images to visible toner images by contact development.
2. Discussion of Background
Two-component or one-component dry developers are in general use for the contact development in electrophotography. Although the one- and two-component developers have their own merits and drawbacks, one-component developers are advantageous over two-component developers because the composition thereof does not substantially change while in use for an extended period of time. In addition, one-component magnetic toners are widely used because they can be employed in both contact development and non-contact development, and can be satisfactorily deposited on a development roller and transferred therefrom to latent electrostatic images formed on a latent-electrostatic-image bearing member for the development thereof.
For example, there is disclosed a one-component magnetic toner with high resistivity for use as a non-contact developer in Japanese Laid-Open Patent Application 58-189646. This one-component magnetic toner comprises a magnetic material containing FeO in an amount of 16 to 25 wt. %. In this one-component magnetic toner, the amount of FeO is limited to the above-mentioned amount in view of the image transfer performance of the toner and the color tone of the obtained images. In this Japanese Laid-Open Patent Application, it is asserted that the fluidity of FeO is good and accordingly the dispersibility of FeO in the toner is also good, so that the image transfer performance of the toner and the color tone of obtained images can be improved.
However, it is considered that there are some difficulties in employing the above-mentioned one-component magnetic toner as it is for contact development. The reasons for this are as follows: (1) Japanese Laid-Open Patent Application 58-189646 asserts that the one-component magnetic toner disclosed therein is suitable for non-contact development, but does not mention anything about the application of the toner to contact development method. (2) The inventors of the present invention have discovered that not all one-component magnetic toners comprising an iron-based magnetic material are suitable for contact development, and the suitability thereof depends upon the structure or composition of the iron-based magnetic material employed therein.